Micro-electromechanical systems (MEMS), such as accelerometers or gyros, often require a gap to be provided between a portion of two layers. This gap may be required, for example, for capacitive functions in a moving structure of the device, self-test excitation or damping of a moving structure.
In such systems, extremely accurate control of the gap dimensions is required, often down to micron level. Furthermore, the gap often needs to be hermetically sealed, generally in a controlled atmosphere, a gas filled environment or a vacuum.
This combined requirement has limited the bonding methods that are currently possible. Current metal bonds based on solders, eutectic or similar process approaches are of limited use because the required thickness of such layers and the metal float phase of the bonding process makes accurate gap control very difficult.
Furthermore, in a number of applications, there is a need to provide an electrical connection between the main structure and, for example, electrodes or a patterned structure on the sealing part. There is also sometimes a need to provide one or more vertical electrical connections through the whole structure. These are sometimes called vertical through conductors, or through-silicon-vias (TSV).
An existing method of creating a gap between two layers being joined together is based on anodic bonding. A problem with this technique, however, is that the bond members must be of different materials with non-matching mechanical or temperature characteristics, in addition to requiring a high electric field during processing. Other methods of bonding include adhesive bonding, glass fit bonding, direct/fusion bonding and eutectic (e.g. AuSn) bonding. However, although these methods are able to create seals, they are not able to provide very accurate gap control.